The synthesis of monochloroacetic acid on an industrial scale is conducted by chlorination of acetic acid, but inevitably dichloroacetic acid, and sometimes a small amount of trichloroacetic acid are formed as well. One thus obtains a crude monochloroacetic acid which is a mixture consisting of monochloroacetic acid, dichloroacetic acid, traces of trichloroacetic acid and unreacted acetic acid. Because of the proximity of the boiling point of MCAA (189.degree. C.) and DCAA (194.degree. C.), it is practically impossible to separate them by distillation. In contrast, it is very simple to hydrogenate this mixture to convert the DCAA into MCAA according to the reaction: EQU CHCl.sub.2 COOH+H.sub.2 .fwdarw.CH.sub.2 ClCOOH+HCl
This hydrogenation is not entirely selective and the retrogradation of MCAA into acetic acid is also observed: EQU CH.sub.2 ClCOOH+H.sub.2 .fwdarw.CH.sub.3 COOH+HCl
This reaction is conducted with a catalyst and a secondary product of acetaldehyde, which has the drawback of generating condensation products.
The hydrogenation of crude monochloroacetic acid also produces heavy compounds such as glycolic acid monochloroacetate (GAMA).
The hydrogenation of crude monochloroacetic acid occurs in the presence of palladium that is deposited on carbon (coal), alumina, or silica. This method is described, for example, in U.S. Pat. No. 2,863,917 and British Patent No. 1,188,745. French Patent No. 2,647,032 also describes the doping of these palladium catalysts with sulfur or sulfurous compounds. French Patent No. 2,046,424 describes a method for the regeneration of these palladium catalysts. Indeed, these catalysts lose their activity while being used, as well as the selectivity, and they generate more aldehydes than heavy products. According to French Patent No. 2,046,424, one oxidizes used catalysts with gaseous chlorine at a temperature between 100.degree. and 150.degree. C. to convert the palladium into its chloride; then, one reduces this with hydrogen or other reducing agents to again produce the metallic palladium.